1. Field of the Invention
This invention relates to an induction heating apparatus, and more particularly to an induction heating apparatus using a digitized control circuit.
2. Description of the Prior Art
The induction heating apparatus, as shown in FIG. 1, basically comprises a heating coil 2 connected to a D.C. source, a resonance capacitor 5 constituting a resonance circuit together with the heating coil 2, a switching element 6, a drive circuit 8 therefor, and a control circuit for turning ON/OFF the switching element 6. Such apparatus, when the switching element 6 is controlled to be ON/OFF, generates in a heating coil 4 a resonance current, thereby generating an alternating magnetic field, at which time a cooking utensil 12 (such as a pot or a frying-pan) of iron or 18-8 stainless or the like disposed in proximity to the heating coil 4 is inductive-heated, thereby heating foods in the cooking utensil 12.
The aforesaid conventional induction heating apparatus, as disclosed in, for example, Japanese Patent Laid-Open No. 58-16493 (1983) has used an analog control circuit controlling ON/OFF of the switching element 6 by use of the time constant of resistance 4 and capacitor 3.
Such conventional control circuit using the resistance-capacitor time constant circuit is larger in a change by time ageing and a temperature change of the time constant, whereby it is difficult to maintain exactly in a set value the on duration length of switching element 6. Accordingly, a problem has been occurred in that the heating output coincides with no set value and the actual heating output is different from materials, formations or the size of cooking utensil 12.
The control circuit using the R-C time constant circuit is difficult to integrate, especially to be a monolithic integrated circuit, which has hindered miniaturization and simplification of the control circuit, in turn miniaturization (especially of smaller vertical thickness) and simplification of the induction heating apparatus as a whole, resulting in a hindrance to the provision of induction heating apparatus inexpensive to produce and less in trouble.
In a case of using the cooking utensil of material, such as aluminum, nonmagnetic and of low resistance, the equivalent resistance and equivalent inductance of heating coil becomes smaller. Hence, when the on duration of switching element 6 is longer, a value of a current Icon flowing in the switching element 6 becomes larger to have a fear of thermal breakdown of the switching element 6.
In order to avoid such fear, conventionally, an AC source to an inverter circuit is detected by a current transformer 11 or the like so that when its current value exceeds the predetermined value, the switching element 6 is off, which has been usual. The above-mentioned construction, in which the input current is detected with respect to the AC source (commercial electric source) put into the induction heating apparatus, has the frequency of one-several hundreds in comparison with the ON/OFF frequency (usually 20 kHz or more) of switching element 6. Therefore, the time period from actual flow of an excessive current to detection thereof becomes longer, whereby in some cases the switching element 6 leads to thermal breakdown.
Now, the inverter circuit at the induction heating apparatus includes a damper diode 7 connected in reversed-parallel to the switching element 6 so that after the resonance duration by the induction heating coil 4 and resonance capacitor 5 finishes and terminal voltage at the switching element 6 drops sufficiently, the switching element 6 is turned on. Just after the switching element 6 is on, however, a feedback current ID flows in the damper diode 7, whereby no current flows in the switching element 6 for the diode duration of flowing the feedback current in the diode 7. Hence, in a case where the control circuit at the induction heating apparatus is digitized merely to detect the on duration of switching element 6 as that of flowing a current therein, it cannot be said to perform output control and overcurrent protection with accuracy.
When the input current value becomes lower than the reference value detected by the current transformer 11, a load is deemed unsuitable, in which just after the heating starts, the input current is smaller so that suitable load is not detectable. Accordingly, since a certain time period is required until the unsuitable load is detected, when the load is actually unsuitable, the power is consumed in vain. Furthermore, the reference value of unsuitable utensil detection is required to correspond to a heating output set value, which will complicate the circuitry when the control circuit is digitized.
Furthermore, in the induction heating apparatus, when the load is smaller to a certain extent than the heating output, in other words, when a small object is loaded, the heating is adapted to stop for protection of switching element 6 or the like. The small object, however, is different in the size due to the condition of a country using the induction heating apparatus or circumstances of a user himself or the like. Accordingly, it is necessary to manufacture the induction heating apparatus which sets the various detection levels of small object. Since the small object detection level actually is set as a ratio with respect to the set value of heating output, so that when the control circuit is digitized, a logical operation circuit is required which converts the set value of heating output into the small object level, resulting in the complicated circuitry, in turn the complicated manufacturing process and a high manufacturing cost.
When the control circuit at the induction heating apparatus is digitized, it is considered that various inconveniences will occur unless the sampling cycle period is set proper. For example, in detection of unsuitable utensil as aforesaid, when the set value of heating output is changed in heating operation, until the next detection timing of input current, in other words, until the next sampling timing, the detected value in the former sampling timing is used. Hence, there is possibility of detecting the unsuitable load by mistake during the above period.
In a case where the control circuit is digitized, it is problematical to what extent the bit number is decided for transmitting the digital signal. For example, if a smaller bit number is taken, a change in width of the ON/OFF duration of switching element caused by a change in the unit bit, or a variation in the input power following the above change, becomes larger, thereby creating inconvenience like noise generation. On the contrary, in a case of increasing the bit number, an A/D converter circuit is complicated to cause a high manufacturing cost and there is possibility of creating the hunting phenomenon in the ON/OFF duration control of switching element.
When the control circuit of induction heating apparatus further is digitized, it is of course considered to make it a monolithic integrated circuit (MIC). A household induction heating apparatus, however, uses the commercial AC power source as full wave rectification. Hence, even when the on duration of switching element is constant, if voltage of commercial power source is different, an amount of a current flowing in the switching element is different, the heating output also being different. Accordingly, it is required to manufacture a control circuit adjustable of the on duration of switching element corresponding to voltage of the commercial power source in use, or a MIC control circuit corresponding to voltage at the same.
Now, there are some induction heating apparatus which is obtainable of the mode of setting a heating temperature to heat the cooking utensil other than the heating mode to set the heating output and heat the cooking utensil. The induction heating apparatus obtainable of such mode is usually constructed such that a thermistor provided at the rear surface of a top plate on which the cooking utensile is placed, detects indirectly a temperature of the content in the cooking utensile.
FIG. 25 shows the relation between an oil temperature T oil and a detection temperature T th in, for example, cooking of deep fry (tempura), in which a graph (a) plots the use of a cooking utensile whose bottom is smaller in thickness and apart at the central portion from the top plate of induction heating apparatus, that (b) shows the same high in thermal conductivity and in close contact at the overall bottom with the top plate, and that (c) whose bottom is thick and, as a whole, in close contact with the top plate. At the time T.sub.A when the detection temperature Tth by the thermistor reaches the set temperature Tref, the oil temperature Toil may, as shown in FIG. 25(c), be considerably lower than the set temperature Tref. Hence, in this kind of conventional apparatus, following method is well known; after the detection temperature Tth at first reaches the set temperature Tref and at the time T.sub.1 after the lapse of time for several minutes (usually 4 to 5 minutes), a buzzer is sounded, or an indicator lamp is lighted, to thereby inform the user that the oil temperature Toil reaches the set temperature Tref. However, some utensils, as shown in FIG. 25(a), may have rised at the oil temperature Toil higher than the set temperature Tref at the time T.sub.A when the detection temperature reaches the set temperature Tref, or some ones, as shown in FIG. 25(b), may almost coincide at its detection temperature Tth with the oil temperature Toil. Therefore, in a case of using such utensil, the conventional induction heating apparatus will cause the power to be wasted.
Lastly, the problem will be discussed when quick heating is carried out when in use of the conventional induction heating apparatus. Namely, in the conventional induction heating apparatus, even when the quick heating is required, was limited to raise its heating output only in a range of the set input power. Hence, release of the restriction of input power is proposed, in which the on duration of switching element becomes very large, thereby occuring possibility of causing thermal breakdown in the switching element.